Various plated steel plates having excellent anticorrosion properties, workability, coating properties, weldability and the like have been developed for use in motor vehicle bodies, electrical household appliances and building materials, and are widely used. In order to stabilize the production quality of these plated steel plates, it is essential to analyze the thickness (deposit value) and the composition (content) of the plating to carry out the process control.
For a steel plate having a plating composed of a content other than Fe, such as a Zn plated steel plate and a Zn--Ni alloy-plated steel plate, the thickness and the composition of the plating can be comparatively easily analyzed using fluorescent X-rays, and apparatus for this type of analysis has been put to practice.
However, analysis of a Zn-Fe group alloy-plated steel plate, the outstanding characteristic feature of which has been noted recently, has been impossible by the ordinary method of fluorescent X-rays because the intesity of the Zn fluorescent X-rays is varied due to the content of Zn or Fe in the plating and the thickness of the plating. In addition, for Fe, a large quantity of Fe fluorescent X-rays are generated from a substrate steel plate, and these fluorescent X-rays cannot be discriminated from the Fe fluorescent X-rays in the plating.
For this reason, the following methods of analyzing the Zn--Fe group one layer alloy-plated steel plate have heretofore been proposed. One method is proposed in Japanese Patent Laid-Open No. 24680/1980, wherein, using a Zn plated steel plate which has been subjected to the Zn--Fe alloying process, a fluorescent X-ray intensity of a metal other than Fe, i.e. that of Zn, is measured by two measuring angles differing from each other. Using predetermined simultaneous equations on the basis of both measured values, the thickness of the plating on the plated steel plate and the degree of alloying (Fe content) are obtained. According to this method, Zn fluorescent X-ray intensities of a sufficiently thick pure Zn sample are previously measured by two measuring angles, and subsequently, a Zn fluorescent X-ray intensity of a Zn alloy-plated steel plate is measured by the same X-ray spectroscope. Analysis is made of the ratio with the pure Zn fluorescent X-ray intensity obtained previously at the respective measuring angles. The fluorescent X-ray quantitative method wherein the measuring angles are varied has been the basic theory of the method of analyzing the fluorescent X-rays published in textbooks from old times and is well known to everyone.
A second method is proposed in Japanese Patent Laid-Open No. 223047/1983, wherein an Fe content in the plating of a Zn--Fe alloy-plated steel plate is obtained from an Fe fluorescent X-ray intensity by a first excitation ray incident angle and the fluorescent X-ray measuring angle, both of which Fe fluorescent X-rays from the substrate steel plate are not substantially detected. The thickness of the plating is obtained from an Fe fluorescent X-ray intensity by a second excitation ray incident angle and a fluorescent X-ray measuring angle, both of which Fe fluorescent X-rays from the substrate steel plate can be detected.
However, since a plated steel plate flows at a speed as high as 100 meter/min, for example on a production line, the plated steel plate is bound to flutter more or less. The influence of this fluttering is received only by the steel plate, so that, according to the former method proposed in Patent Laid-Open No. 24680/1980, the analyzing accuracy is necessarily deteriorated.
In general, the thickness of the plating of the alloy-plated steel plate is as thin as 20-30 g/m.sup.2 (about 3-4 micrometer). If the fluorescent X-rays are strong enough to accurately measure a metal in the plating by the latter method proposed in Patent Laid-Open No. 233047/1983, then it is impossible to excite only the thin plated layer, and the substrate metal as well as the thin plated layer are bound to be excited. In consequence, both the fluorescent X-ray intensities of Fe in the plating and Fe in the substrate steel plate are measured, so that an accurate analysis cannot be made. Further, when an on-line systemizing is intended, with a low incident angle=measuring angle=5.degree. adopted in the first X-ray optical system, construction of such proposed apparatuses as described above have been impracticable, because of the size of an X-ray tube, construction of an X-ray spectroscopic system, a protective cover of an analysis meter and the like.
In consequence, both the methods which have been proposed are disadvantageous. Therefore, a method of chemical analysis has been relied on wherein only the plated layer is dissolved and removed by electrolysis or a suitable acid. The thickness of the plating is obtained from the value of removal, and an Fe content in the plating is obtained through chemical analysis of the value of Fe in the solution. However, it is extremely difficult to dissolve only the plated layer without dissolving the substrate steel plate. It requires a considerably high skill level and a long period of time to conduct this chemical analysis method. Moreover, this analysis is a destructive analysis to collect samples from a product and the on-line systemizing cannot be attained, thus presenting such a disadvantage that the reflection of measured results to the process control is delayed to a great extent.
It is very difficult to analyze even the Zn--Fe group of a one layer alloy-plated steel plate as described above. For a Zn--Fe group two layer alloy-plated steel plate having a first layer with a Zn--Fe alloy plating and a second layer with an Fe plating having Fe as the chief content, the problem becomes further complicated and measurement with high accuracy becomes extremely difficult.
In an invention analogous to the present invention, apparatus for continuously inspecting a quality of steel plate has been proposed in Japanese Patent Laid-Open No. 17695/1975. The apparatus thereof comprises: means for causing characteristic X-rays and white X-rays to irradiate a continuously moving steel plate at a predetermined angle; means for detecting diffracted X-rays which have wavelengths satisfying a condition of Bragg and fluorescent X-rays from this irradiating point; and means for analyzing these detection signals to sense intensity values of respective aggregate structures, elements, etc. However, this invention is different in object and constitution from the present invention, and moreover, the detection of the thickness of plating is not conducted.
The present invention has been developed to obviate the disadvantages of the prior art and has as one object the provision of a method of measuring the thickness and the composition of an alloy plating, wherein the thickness and the composition of a one layer alloy plating which includes a metal identical to a substrate metal can be measured simultaneously and non-destructively.
The present invention has as another object the provision of a method of measuring the thickness and the composition of an alloy plating, wherein the thickness and the composition of an alloy plating having two layers different in composition from each other and which include a metal identical to a substrate metal, can be measured simultaneously and non-destructively.